That said, I don't know that I totally agree with Sam. Having spent time in EQ, you did not generally go outside of your guild at any time for raiding, unless you were looking to join another -- or get punted from your current one if they caught wind of it. In WoW, I've seen guild alliances and even open-to-all alt runs posted on the forums featuring members from many established guilds. But I know that there have been days when I dreaded another PUG due to horrendous tactics and playing. Bad PUGs are the fastest way to get me turn to only running with my guild for a little while.
On the other hand, I know many small guilds don't necessarily like folks running with other guilds because they are concerned that their members are going to be poached by larger guilds. But for every single one of these types of insular guilds, there is another that is part of a raiding alliance, with their members mingling freely with other guilds.
What do you think? Do you, like Sam, wonder why people are so utterly devoted to playing and raiding with only their guild? Or do you also see the alt runs and PUGs and think that there's a nice balance in WoW of being able to play with your guild as well as with other people? Does your guild have a policy of "don't run with any other guild" or are you in one of the many different guilds that actually has a raiding and grouping alliance?